1. Field of the Invention
This invention relates to an optical information medium such as read only optical disk and optical recording disk.
2. Prior Art
Optical materials most typically used for light-transmitting layer and the like of an optical information medium are polycarbonate- and polymethyl methacrylate-based materials in view of their favorable moldability, transparency, price, and the like. These materials, however, suffer from insufficient abrasion resistance, and their high electric insulation invites high susceptibility for electric charge, and a large amount of dust is likely to become attached to the surface of the medium during their storage or use to result in errors in the recording and reading of the optical information.
A countermeasure generally taken for such problem is application of a transparent, scratch-resistant hard coat on the light-transmitting layer of the medium. The process most popularly employed in such case is coating of a curable compound having at least two polymerizable functional groups such as acryloyl group in the molecule on the light-transmitting layer followed by curing with UV or other active energy beam to thereby form a protective layer. Although the protective layer formed from such UV-curable resin may be superior in abrasion resistance compared to the surface of the resin such as polycarbonate or polymethyl methacrylate, the level of the abrasion resistance achieved is limited to a certain level and such level is not the level of the scratch resistance sufficient for use in the optical information medium. In addition, these hard coatings are provided only for the purpose of imparting the surface with the scratch resistance, and a smudge-proof surface which is resistant to attachment of dust or oil mist in the air, fingerprint, and the like is not expectable.
Also proposed is application of a hard coat imparted with anti-static property for prevention of dust attachment as well as sufficient scratch resistance on the substrate on the side of the recording/reading beam incidence. For example, Japanese Patent Application Laid-Open No. (JP-A). 239946/1985 and JP-A 276145/1986 propose addition as an antistatic agent of a cationic amine, an anionic alkylbenzene sulfonate, a nonionic polyol, or ethylene oxide of an alkylphenol, and amphoteric imidazoline or alanine metal salt. JP-A 173949/1991 proposes addition of a lauryl compound, and JP-A 80267/1992 proposes addition of thiocyanic acid and an anionic surfactant containing alkylene glycol chain. These surfactants all have smudge proof effects for inorganic substances such as dust. These methods, however, are substantially ineffective in preventing organic smudges such as fingerprint and oil mist. Furthermore, the surface abrasion resistance of the light-transmitting layer in these proposals is equivalent or inferior to that of the conventional hard coats formed by using a UV-curable resin, and the scratch resistance sufficient for practical use is not at all realized.
A hard coat having smudge-proof properties for organic contaminants is proposed in JP-A 110118/1998 wherein the hard coating material used has a non-crosslinked fluorosurfactant kneaded therein. The hard coat obtained by this method, however, is insufficient in water- and oil-repellency since only a small part of the fluorocompound is exposed to the hard coat surface. When the amount of the fluorosurfactant added to the hard coating material is increased in order to secure sufficient water- and oil-repellent function, the resulting hard coat will suffer from reduced hardness, poor optical properties due to leaching of the excessive surfactant, and handling inconvenience.
As a countermeasure for such problem, JP-A 213444/1999 proposes coating of a fluoropolymer on the surface of the conventional optical disk substrate comprising polycarbonate or the like, and in this application, the water- and oil-repellency is imparted by coating the fluoropolymer on the resin substrate of the opt cal disk. In contrast to the method wherein a lubricant or the like is kneaded in the hard coating material, the water- and oil-repellent compound of this application is exposed to the entire surface of the hard coat film and sufficient smudge-proof properties are _thereby realized. The fluoropolymer of this method, however, suffers from extremely poor adhesion to the underlying substrate since the fluoropolymer is only physically adsorbed to the underlying substrate by van der Waals force, and the surface treatment with the fluoropolymer is associated with a serious problem of poor durability. JP-A 187663/1994 proposes coating of an acrylic resin surface with a water- and oil-repellent compound by coupling reaction. In this method, a smudge-proof film exhibiting higher adhesion to the underlying surface compared to the JP-A 213444/1999 is provided by coating a water- and oil-repellent compound containing silyl group on the surface of an acrylic resin containing a hydrophilic substituent.
However, the acrylic resins disclosed in the JP-A 187663/1994 are required to contain an adequate amount of hydroxyl group in the polymer chain to thereby enable adsorption of the water- and oil-repellent compound by coupling reaction. This inevitably results in the limited choice of the acrylic resin. In addition, density of the hydroxyl group should be increased to achieve sufficient adhesion between the hard coat surface and the water- and oil-repellent compound, and this may result in the reduced hardness of the hard coat. Alternatively, the hydrophilicity of the hard coat surface may be raised by high energy beam treatment such as plasma or corona discharge treatment. Such treatment, however, is not sufficient in effectively enabling the coupling reaction with the water- and oil-repellent agent, and the satisfactory adhesion is less likely to be achieved solely by this method.
JP-A 203726/1999 discloses a method for improving the scratch resistance of the surface of a resin light-transmitting layer. In this method, two or more inorganic material layers of SiN or SiO are formed by vapor deposition such as sputtering to a total thickness of approximately several hundred nm on the light-transmitting layer comprising a UV-curable resin. However, it is quite difficult to obtain a scratch resistance of practically acceptable level by forming an inorganic film of such thickness.